The present invention relates to electromagnetic apparatus and to elements in the magnetic circuits of such apparatus for reducing eddy currents.
In general, electromagnetic apparatus including machines for converting between the mechanical and electrical energy are well known, e.g., motors and generators. Such conversion machines typically comprise a rotor and stator arranged for relative motion, e.g., rotation or linear displacement. In an electric motor, varying magnetic fields are typically generated by the stator. These fields interact with a magnetic field associated with the rotor, imparting motion to the rotor, e.g., causing the rotor to turn. Conversely, in a generator, the rotor is driven by a mechanical energy source (e.g., a motor or engine) and generates a magnetic field (using either permanent magnets or windings), which interacts with windings formed on the stator. As the magnetic field of the rotor intercepts the windings on the stator, an electrical current is induced in the stator windings. The induced current is typically applied to a bridge rectifier, regulated, and provided as a DC output power signal from the generator. The output of the bridge rectifier may be applied to an inverter to provide an AC output power signal. Electromagnetic apparatus includes machines that operate with direct current, single phase alternating current, or polyphase alternating current. Such apparatus may operate with relatively continuous motion (e.g., a turbine driven generator) or intermittent motion (e.g., a stepper motor or a linear positioning motor).
Examples of portable engine driven generators (gensets) are described in U.S. Pat. No. 5,929,611 issued to Scott et al. on Jul. 27, 1999; U.S. Pat. No. 5,625,276 issued to Scott et al. on Apr. 29, 1997; U.S. Pat. No. 5,705,917 issued to Scott et al. on Jan. 6, 1998; U.S. Pat. No. 5,780,998 issued to Scott et al. on Jan. 14, 1998; U.S. Pat. No. 5,886,504 issued to Scott et al. on Mar. 23, 1999; U.S. Pat. No. 5,900,722 issued to Scott et al. on May 4, 1999; and U.S. Pat. No. 5,929,611 issued to Scott et al. on Jan. 27, 1999; all commonly assigned with the present invention. Portable power conversion systems find particular utility as power sources and vehicular battery charger/jump start units. Power may be used for lights, small appliances, or in connection with recreational vehicles. These uses typically arise at construction or camping sites.
Generators that use permanent magnets to develop the requisite magnetic field tend to be lighter and smaller than generators that use field windings for that purpose. However, the power supplied by a permanent magnet generator has historically been difficult to regulate or control. The voltage provided by a generator may vary significantly according to the rotational speed of the rotor (e.g., measured in RPM). In addition, this output voltage tends to vary inversely with the current delivered so that, as the current delivered to a given load increases, the output voltage provided by the generator across the load decreases.
Rotors employing high energy product magnets and consequence poles are also known. Such rotors are described in, for example, the aforementioned U.S. Pat. No. 5,705,917. That rotor employs a body of soft-magnetic material carrying a plurality of permanent, high energy product magnets. These magnets may have a flux density of at least on the order of five kilogauss and may be suitably formed of a rare earth alloy such as neodymium iron boron, or samarium cobalt. These magnets are disposed on a peripheral surface of the body of the rotor, mounted in recesses (also called insets) formed in the rotor surface. The portions of the soft magnetic rotor body between the recesses form respective consequence poles.
Rare earth materials tend to be extremely expensive. Accordingly, it is desirable to minimize the amount of these materials used. However, at the same time, it is desirable to generate relatively high flux densities. In the preferred embodiment of the aforementioned U.S. Pat. No. 5,705,917, the magnets employed are relatively thin (e.g. about {fraction (1/10)} of an inch thick) and present a relatively large area (e.g., about xc2xe of an inch by about one inch) to minimize the amount of high energy product magnet used. By maximizing the area of the permanent magnet relative to the area of the consequence pole, a desirable total flux is achieved with a smaller diameter core. Less weight and less magnetic material are typically used with a smaller diameter core.
In addition, it is also known that permanent magnets are subject to demagnetization when exposed to high temperatures and/or high magnetic flux. Heating permanent magnets to near their Curie temperature can cause demagnetization and result in loss of other performance characteristics. Permanent magnets installed in conventional rotors are susceptible to physical damage and/or demagnetization from eddy current heating and heat associated with mechanical vibrations induced from air gap harmonics. Such heat may be particularly evident at the permanent magnet surface (e.g., during exposure to flux changes across the air gap). In addition, permanent magnets on rotors are subject to the electromagnetic fields generated by current induced in the stator windings. Shorts in simple turn-to-turn or phase-to-phase stator windings may produce dramatic heating of magnets installed in conventional rotors and thus lead to demagnetization.
Attempts have been made to a avoid demagnetization of rotor magnets due to electromagnetic fields associated with current flow in the stator windings. For example, U.S. Pat. No. 5,298,827 issued to Sugiyama on Mar. 29, 1994 describes a permanent magnet dynamoelectric machine rotor in which a ferromagnetic material (e.g., high magnetic permeability soft iron) is attached to the outer magnetic pole face of each of the permanent magnets. The magnetic flux generated by the stator windings passes through the ferromagnetic material so that demagnetization of the permanent magnets caused by flux generated by the stator windings can be reduced.
The flux path or magnetic circuit formed in a conventional electromagnetic machine is generally supported by high permeability materials formed as stacks of laminations electrically insulated from each other. Electrical insulation interrupts eddy current flow, reducing energy losses and avoiding localized resistive heating in the regions of the eddy currents. The flow of flux between laminations is inhibited. Because conventional laminations are generally planar, electromagnetic machine design has been limited to implementing flux paths in two dimensions through laminated members.
A pole for use in an electromagnetic apparatus in an implementation according to various aspects of the present invention has a magnetic circuit that includes a flux-passing portion for passing magnetic flux and a cap. The flux-passing portion has a face through which at least some of the magnetic flux passes and to which the cap is fixed. The cap is made of a permeable material and is formed to impede eddy currents in the cap.
By forming the cap of material having relatively low conductivity (e.g., an insulator or substantially nonconductive material), energy losses in eddy currents may be reduced or avoided. The heat generated by such eddy currents may be reduced or avoided. The detrimental consequences of local heating (e.g., tempering, expansion, convection, infrared radiation, etc.) due to such eddy currents is consequently reduced or avoided. In one implementation, the cap is formed from a particulate permeable material disposed in a binder. A method of forming the cap may include the steps of: (a) providing a powder comprising particles having magnetically soft permeability each coated with a binder, the binder having lower conductivity than soft iron; (b) molding the powder to form the cap; and (c) heating the powder so that the powder forms a matrix of soft iron particles disposed in a solid formed from the binder.
The cap may have a top and bottom surface joined by a shoulder to provide a distribution of magnetic flux through the shoulder that is less concentrated than the flux distribution through a 90-degree corner. When concentrations of flux are distributed, eddy current losses may be lower and heating due to eddy current losses may be reduced or avoided.
The pole may be incorporated into any electromagnetic apparatus to increase an operating efficiency, for example, a motor, a linear motor, an alternator, a generator, or a portable engine-driven generator.
An electromagnetic apparatus according to various aspects of the present invention has a flux circuit supported at least in part by a permeable material that is formed to impede eddy currents. In an implementation for rotary motion, the apparatus may include a first pair of permanent magnet poles spaced apart on a circumference centered on the axis of rotation. Each pole may include a permanent magnet and a cap as discussed above. The implementation may further include a second pair of poles implemented to face the first pair to complete a flux circuit. The second pair of poles may be provided in magnetic communication with a support and spaced apart on a second circumference centered on the axis of rotation. The support may be formed at least in part by a permeable material that is formed to impede eddy currents. When a winding is provided between the second poles to pass through the flux circuit, rotation of the first poles with respect to the second poles may give rise to a current in the winding. When the second poles are spaced apart in separate parallel planes, the winding may be formed in a plane generally between the second poles and encircling the support within a relatively small range of radii from the axis of rotation.
By forming the support of a permeable material that impedes eddy currents, the electromagnetic apparatus operates at higher energy efficiency. Localized heating with consequential dimensional changes can be avoided.